In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1).
In this LTE and successor systems of LTE (referred to as, for example, “LTE-advanced,” “FRA (Future Radio Access),” “4G,” etc.), a radio communication system to support inter-terminal communication (D2D: Device-to-Device) is under study. In inter-terminal communication, a user terminal discovers other user terminals (discovery) without involving radio base stations (that is, directly), and communicates with these other user terminals.